High-frequency signaling



Dec. 11, 1928.

B. W.' KENDALL HIGH FREQUENCY SIGNALING Original Filed April 21. 1919 4 Sheets-Shee t 1 1W N l \1 Mr W.

//7ve/7 for.- Burfon W Kenc/d/ Dec.11, 1928. 1,694,473

E. w. KENDALL HIGH FRE UENCY SIGNALING Original Filed April 21, 1919 4Sho6js-Sheot 2 vemor; I 5 /7 W Kane/a Dec. 11, 1928. 1,694,473

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HIGH FREQUENCY S IGNALING Original Filed April 21, 1919 4 Stunts-Sheet 3 50/"20/7 W Kendd/ Dec. 11, 1928.

HIGH FREQUENCY SIGNALING Original Filed April 21, 1919 4 Sheats -Sheat 4 Patented Dec. 11, 1928.

UNITED STATES PATENT OFFICE.

BURTON W. KENDALL, OF EAST ORANGE, NEW JERSEY, ASSIGNOR TO WESTERN ELEC- TRIO COMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

HIGH-FREQUENCY SIGNALING.

Application filed April 21, 1819, Serial. No 291,449.

This invention relates to high frequency signaling in which signals are transmitted as variations or interruptions of a carrier Wave.

According to the homodyne method of carrier wave reception for telephony the amphtude of the detected signal is intensified by combining the received modulated carrier Wave oscillations with locally generated oscillations of the carrier frequency. The 10- cally generated oscillations will herein be termed reinforcing oscillations to differentiate them from the received carrier oscillations. If the carrier oscillations differ in frequency from the reinforcing oscillations, beats of the difference frequency are produced and may be manifested as a sound or noise which is superimposed upon the signal being received,- thus interfering With a clear distinctreception of the signal.

The present invention has for an object to provide a method and means for insuring that the carrier oscillations and the reinforcing oscillations shall have substantially the same frequency. Another object of the 1nvention is to provide a multiplex carrier telephone system. A further object of the invention is to provide a homodyne signaling system in which the phase of the reinforcing oscillations generated at the receiving station may be regulated with respect to that of the received carrier oscillations. A still further object of the invention is to provide means for generating a plurality of different frequency carrier Waves, of suitable ampliture for multiplex transmission from a single control or base frequency wave.

According to this invention control or base frequency oscillations are generated at one station. The waveform of a portion of the control oscillation energy is distorted and one or more harmonics are selected from the resulting wave and then amplified to serve as carrier waves. A particular carrier Wave so produced is modulated in accordance with both the signal or other teledynamic impulses which it is desired to transmit and the original or undistorted control frequency oscillations. The transmitted carrier wave thus carries simultaneously, the signal and the control oscillations. At the receiving station the received carrier Wave is detected and the resulting detected signal and control oscillation components are separated. The

wave form of the control oscillation compo Renewed September 16, 1927.

oscillations are derived, the carrier and re .inforcing oscillations w1ll always be of equal frequency although the absolute value of the frequency may vary in consequence of variations in the control frequency. This scheme of frequency control may be embodied in a multiplex system by causing the harmonic producing means at the transmitting station to produce a plurality of harmonic frequency carrier waves from the distorted control Wave and then transmitting each of these carrier waves modulated both in accordance with the control wave and its individual message wave,

as previously described.

Where the receiving stations are comparatively close to each other, instead of transmit ting the control oscillations as modulations of each individual carrier, the control oscillations may be transmitted continuously as modulations of a single carrier. The wave form of the detected control oscillations may then be distorted and reinforcing oscillations of each of the transmitted carrier frequen cies derived therefrom. This enables a single carrier wave to t 'ansmit the control oscillations and permits transmission of the remaining carrier waves modulated in each in accordance only with its individual message. The unmodulated carrier waves may be suppressed in each instance to increase the efliciency of the transmitting station and to lessen the probability of crosstalk. One side band of the modulated carrier frequency may be suppressed to decrease the range of the transmission band required for a single message or transmission and to correspondingly increase the number of simultaneous non-interfering transmissions.

Further objects and advantages of the invention will appear from the following detailed description taken in connection with the accompanying drawing, in which Fig. 1 illustrates a ran m tt ng a on, a F

2 the corresponding receiving station of a multiplex sign aling system according to this invention, and Figs. 3 and 1 illustrate the transmitting and receiving stations respectivelyof a modified system.

Referring to Fig. 1,'the oscillation generator 0 comprises a three-element v cuum tube, preferably highly evacuated, the input circuit of which is electrically connected to the output circuit by the oscillatory circuit formed by coils 2 and 3 and variable condenser 5. A source 4t supplies space current to the tube. By means of the variable condenser 5 of the oscillatory circuit, the frequency of the oscillations produced by the generator may be given any desired value. This oscillation generator which produces control oscillations is connected to the input circuit of amplifier A of the vacuum tube type. The impedance control element 6 of this amplifier is negatively charged with respect to the cathode by source 7. The space current of the amplifier is supplied by source 8 tl'irough the impedance 9. Cathodes 10 and 11 of the oscillation generator and an'iplifier respectively, are heated by current from the common source 12. Amplified oscillations in the output circuit of the amplifier A. are supplied by a transformer connection to the input circuit of the harmonic generator or producer G which consists primarily of a three-element vacuum tube.

The tube of the harmonic generator G is overloaded so as to produce a maximum distortion in the output circuit of the wave form of the repeated control oscillations supplied to its input circuit. This is done by applying to its input terminals an alternating potential such that the maximum value carries the operation of the tube beyond that portion of its characteristic which is substantially linear, thus substantially distorting the Wave form ofthe applied alternating potential as repeated in the output circuit. This applied alternating potential may be such that the maximun'i positive value will exceed that nee essary to cause the tube to become saturated and the maximum negative value will exceed that negative value required to reduce the output currentsubstantially to zero, or one of these conditions may exist alone to the substantial exclusion of the other.

Since a distorted or non-sinusoidal periodic wave may be analyzed into a pure sine wave component of the frequency of the distorted wave and a plurality of harmonic components of higher frequencies. it remains to select the particular harmonic components which it is desired to utilize as carrier waves.

The cathode 13 of the harmonic generator is heated by current source 14 and the space current of the tube is supplied by source 15 through the impedance 16. The condenser 1? in shunt to the impedance 16 and source 15 provides a low impedance path for high frequency currents. The harmonic generator is coupled by means of a transformer to an amplifier A which is similar to amplifier A, previously described. The output circuit of amplifier A includes a plurality of tuned loop circuits 18, corresponding in number to the number of harmonics which it is desired to select and each circuit 18 is tuned by means of its variable capacity element 19 to the particular harmonic which it is to select. Coupled by a. transformer to one of the tuned loop circuits 18 is an amplifier A similar to the amplifiers described and having an input circuit 20 coupled to circuit 18 and similarly tuned. The input terminals of each amplifier A are connected by a potentiometer coupling to the terminals of the condenser of the tuned circuit 2O. This permits the particular harmonic selected to be amplified to the desired degree and the amplitudes of the various harnjionics can thus be made of -.=.aubstantially the same n'iagnitude if desired. Amplifier A is connected by a transformer to a band filter l which passes a narrow band including the selected harmonic with substantially no at tenuation, and extinguishes currents of frequency diilering' considerably therefrom. Filter F terminates in a potentiometer coupled to the common input path of abalanced modulator M of the type disclosed in U. S. patent to Carson No. 1,343,306, dated June 15,1920.

The output circuit of amplifier A is also coupled to the input circuit of an amplifier A the latter being connected through a band filter F and another amplifier A. to a potentiometer in the common input path of modu later Filter F, passes a narrow band of frequencies including the control oscillation frequency. A circuit 21, illust a'ted as a telephone circuit but which may be taken as rep-' resentative of any low frequency current line transmitting signal currents or other telcdynamic electrical variations, is coupled to the divided inputcircuit of modulator M. The modulator M is coimected by a band filter F to high frequency circuit L which may be an antenna or a transmission circuit. The charaeteristic of modulator M is such that it transin its no energy to filter F except at such times as variation currents traverse circuit 21 and set up corresponding variation electroniotive forces in the divided input circuit of the modulator. During the existence of these variation currents the modulator transmits to filter F oscillations of the frequency selected by tuned circuits 1S and 20 modulated both in accordance with the variation currents of line 21 and the control oscillations supplied from amplifier A The other tuned loop circuit 18 is similarly connected by an amplifier and band filter to a modulator M, similar in every respect to nodulator M. It will of coursebeunderstood that each tuned looo circuit 18 selects a par ticular harmonic and that the circuit connecting that tuned loop circuit with its individual modulator is designed to transmit the particular harmonic selected. A low frequency circuit 22 similar to circuit 21 is associated with modulator M,. A band filter F connects modulator M to circuit L. Control oscillations are supplied from filter F to modulator M by a circuit identical with that which connects filter F to modulator M.

Preferably, each of the band filters F F transmits only one side band of the modulated wave supplied by its individual modulator M or M,. In other words, if the frequency of the carrier oscillations supplied to modulator M is 30,000, so that the resulting modulated Wave comprises a band of frequency components extending equally above and below this frequency, either the portion of the band above the carrier frequency or the portion below it is suppressed by the filter F and the filter is so designed as to pass only the remaining side band.

Only two low frequency circuits 21, 22 and the corresponding high frequency transmission channels are illustrated, but it is to be understood that as many more as desired may be provided, each being designed to select, modulate and transmit an individual carrier frequency wave.

Referring to Fig. 2, a high frequency receiving circuit L, which may be either an antenna or a transmission circuit, is con nected by unidirectional amplifiers UA to two similar receiving channels by band filters F and F effectively in parallel with respect to circuit L. Band filter F is designed to pass the same band of frequencies as filter F at the transmitting station. Filter F supplies modulated oscillations from circuit L to detector D which may be of the balanced type and which is coupled by loop circuit 23 to filters F and F respectively. Upon detection the received modulated carrier wave passed by filter F yields in the output circuit of the detector a current component corresponding to the variation currents of circuit 21. Filter F is designed to pass a low frequency band, within which these currents fall, to receiving instrui'nent 2st illustrated a t8lCpllOIlG receiver but which, as will be understood, may be any signaling or other teledynamic receiving device, depending upon the nature of the variable currents or impulses of circuit 21 and the operation which the receiving instrument is designed to perform. In the case of speech transmission, the filter F will preferably suppress substantially all currents of frequencies above 2500 cycles and will readily transmit all frequency currents below this value. The detected carrier wave also yields a component of the control oscillation frequency and this is transmitted .from circuit 23 through a filter F T which is similar in electrical clniracteristics t0 filter F,. From filter F the detected control oscillations pass toa harmonic producer G, Connected to the output circuit of harmonic produced G, is an amplifier A having an input circuit tuned to the frequency of the carrier oscillations selected by circuit 20 of Fig. 1. Amplifier A transmits the selected harmonic reinforcing oscillations to a circuit in.- cluding a phase regulating device which consists of a variable inductance 25, a variable capacity 26, and a variable resistance 27. The reinforcing oscillations after phase regulation. are passed to amplifier A having a potentiometer input circuit and from the amplifier, amplified reinforcing oscillations are supplied to the common input path of detector D to be combined with the received carrier oscillations.

The phase regulating device is variable and it is accordingly possible to secure any desired phase relation between the received carrier oscillations and the reinforcing oscillations. For some purposes the amplification is increased by bringing these two sets of oscillations into phase agreement.

The reinforcing. oscillations combine with the received oscillations to produce intensified control oscillation components and signal current components in the output circuit of the detector.

The element G serves both as a harmonic generator or producer and a power limiting device. The potential of its grid or impedance control element is maintained positive with respect to the cathode by a source of unidirectional current. The positive grid attracts the electrons emitted by the cathode and when the potential supplied to the device G, reaches a sufficiently high positive value, the electrons will be attracted to the grid to such an extent that no further increase in the space current in the cathode-anode circuit occurs. The minimum value of the space current in device G, being zero, it is evident that the power delivered by this tube is limited. This power limiting action is desirable in order to prevent singing in the circuit F,, G,, A and A which couples the output circuit of the detector to the input circuit. Any other form of power limiting device may be employed Without departing from this feature of the invention.

Filter F and the apparatus .in the receiving channel associated therewith are similar to filter F and its associated channel, previously described, except that the carrier frequency received is different. The tuning of the various filters and circuits of this channel bears the same relation to that of the trans mitting channel of circuit 22 as the corresponding values of the channel of filter F bear to those of the transmitting channel of circuit 21.

Since the transmitted carrier oscillations and the reinforcing oscillations are deriv d from a common. source andv since both are the same harmonic of the oscillations supplied by that source,'it'folloivs that the carrier frequency and the reinforcing frequency will always be identical, which is very desirable for the proper reception of telephonic signals. It is to'be noted that the selecting devices, i. e., tuned circuits and filters employed, will properly differentiate between the various frequency currents involved, even though the control frequency and its multiple or harmonic are subject to small variations.

Referring to Figs. 3 and 4, which illustrate the sending and receiving stations respectively of a modified system, the various oscillators, amplifiers and harmonic generators are similar to those described in connection With Figs. 1 and 2. The control oscillations produced by oscillation generator 0 are amplified by amplifier A, which transmits an1- plified control oscillations to-the input circuit of a harmonic generator G The several carrier oscillations produced by harmonic generator G are amplified by an amplifier A the output circuit of Which comprises two tuned circuits 28 and 29, each "of Which is tunedto a different one'of the harmonics supplied by the harmonic generator G Carrier oscillationssupplied by the tuned circuit 28 are amplified by an amplifier similar in every respect to'aniplifier Ar and are trans mitted by means of filter F corresponding to filter F of 1, to the common input circuit of a balanced modulator M Control oscillations are also transmitted from ampliiierA through an amplifier and filter corresponding respectively to amplifier A and filter F, of the system of Fig. 1. These control oscillations are transmitted by a transformer '30 to the divided input circuit of modulator M 'Modulaitor M has a characteristic similar to that of modulator M and accordingly transmits enei'rgy at, such times as variation electromotive forces are impressed upon its divided input circuit and at all other times transmits substantially no energy. Oscillations of the carrier frequency supplied by circuit 28, modulated in accordance With the control frequency, Will therefore be continuously transmitted through filter F amplifier A. and filter F to the high frequency circuit L A low frequency signaling or variation current circuit 211 similar to circuits E21 and 22, is connected by loop circuit and divided traiisformer 33 to the'divided input circuit of modulator M Wh encver variation currents traverse circuit 31, carrier oscillations mo l i lated in accordance with these variation currents will also be transmitted to circuit L,.

A second low frequency circuit 3-il is connected to the divided input circuit of a modulator M upon. which carrier oscillations selected by circuit 2-9 are impressed. The cir- ,circuit 3d cause the modulator cuits connecting circuit 29 to modulator M are like those connecting circuit 28 to modulator M with the exception that their tuning and transmission range is designed to select and transmit a different harmonic frequency carrier Wave, namely, that seclected by circuit 29. TTOWQVO/l', the control frequency oscillationsare not supplied to modulator M, and accordingly thecarrier Wave impressed upon its input circuit is supn'cised except when variation currents in to transmit pure modulated Waves tocircuit L,. The modulator M connected. to circuit L by a circuit similar to that connecting modulator M to circuit L The iilter F, is designed to suppress currents of the order of frec uencies of those occurring in circuit 31, so as to prevent overloading, amplifier A to which it is connected. A band filter F connects ampliiier A, to circuit L and serves to transmit only the band of frequencies corresponding to the pure modulated carrier Wave and if desired, it may be designed to transmit one side bandonly of the pure modulated Wave.

As was explained in connection with Fig. 1,

the number of channels may be made much larger, each additional channel being similar in every respect to the channel of circuit 84, with the exception that each channel would be so designed as to sel ct, modulate and transmi a particular carrier Wave individual to thatchannel.

In the sending arrangement illustrated in Fig. 3, one channel serves to continuously modulate its carrier Wave in accordance With control oscillations and to transmit the "resulting modulated carrier Wave to circuit L The remaining channels (of which that of circuit 34lonly is illustrated) transmit individual carrier Waves modulated only in accordance With the currents of their respective variation circuits 3%, and suppress their respective carrier Waves when the'corresponding variation current circuits are idle.

Fig. which operates in conjunction with "the sending station of Fig. 3. A high frequency re ceiving circuit L which may be an antenna or a transmission circuit, has connected thereto through unidirectional amplifiers Uri,-

band filters l7 and F corresopuding to the band iilers 1mmeihutely coin. ctcdijo the (.l1'- cuit L, of Fig. 3. Filter if which transmits the same band of irequencies as filter 1%,, receives the continuously modulatedcarricr wave energy transmitted from filter 1F TL,- 1 1V1; and transfers it successively LO detector 1),,

l illustrates the receiving station oscillations of each of the different carrier frequencies are continuously produced. Reinforcing oscillations of its received carrier frequency are continuously supplied to detector D over a circuit arrangement substantially identical with that connecting the output circuit of harmonic generator G to the input circuit of detector D in the receiving system of Fig. 2. Harmonic generator G also serves to produce reinforcing oscillations of the carrier frequency received by filter F and to cause them to beimpressed upon detector D associated with filter F As many additional receiving channels may be provided as there are additional transmitting channels 34, each being similar in themrangement of its parts to the receiving channel of filter F and having its electrical. values correspondingly assigned with relation to the particular carrier frequency individual thereto.

Unidirectional amplifiers are of particular advantage in the present system to prevent reverse transmission. Amplifier A of Fig. 1 prevents the carrier frequency applied to modulator M from reacting upon the output of filter F, and hence upon the input circuit of modulator M .In a similar manner the unidirectional amplifiers UA in the receiving channels prevent transmission of the reinforcing oscillations to the high frequency receiving circuit.

The homodyne method of signaling is of course not limited to telephony but is equally applicable for other teledynamic transmission. The drawing illustrates on e-way transmission only, but it should be understood that the invention is applicable as well to two-way signaling. It is also possible to widely separate the various transmitting and receiving stations. For example, low frequency cir cuits 21 and 22 may be of considerable leiigth and may be subscribers lines terminating in a central exchange at which the high frequency circuits terminate. The system of Figs. 1 and 2 particularly lends itself to separated high frequency station operation since the high frequency receiving channels have no other association than connection to the common transmission line L. The sending stations each require a high frequency connecting circuit to the harmonic generator which serves as the common source of carrier oscillations.

Various features of the systems disclosed herein are claimed by applicant in other copending applications. The homodyne method of reception is disclosed and claimed in U. S. patent to Kendall, No. 1,330, 471, patent ed October 10, 1920. The harmonic generator is disclosed and claimed in U. S. patent to Kendall, No. 1,446,572, granted February 27, 1923. The derivation of the various carrier and reinforcing frequencies'as harmonics of a single control frequency is disclosed and claimed in application Serial No. 130,350, filed November 9, 1916, of which the present application is in part a continuation.

It will of course be understood by those skilled in the art that the specific circuits herein shown and described are not only illustrative of particular arrangements that may be employed, and that the details of the circuits and the relative arrangement and position of the various elements may be materially varied from what is set forth, without departing from the spirit and scope of the invention. Only such limitations are to be imposed on the invention as are indicated in the appended claims.

lVhat is claimed is:

1. In multiplex signaling the method of producing carrier Waves for a plurality of different message transmissions and of fixing the relative frequencies and amplitudes of said carrier waves, which method comprises producing control oscillations, deriving therefrom different harmonic oscillations, selecting the harmonic oscillation waves of the desired carrier wave frequencies and separately amplifying each of said selected harmonic oscillation waves to give said Waves the desired relative amplitudes.

2. In a multiplex transmission system, a source of control frequency oscillations, means for deriving from said control oscillations different harmonic frequency oscillations, and means for separately amplifying said harmonic frequency oscillations to substantially the same magi'iitude.

3. In a system for the simultaneous transmission of a plurality of indications over the same medium, means for supplying carrier waves comprising a common Wave generator, means to derive therefrom a plurality of Waves of different frequencies, including frcquency-selective circuits for selecting individual waves, an individual amplifier associated with each of? said circuits for ainplifying the wave of: respective frequency, means associated with each amplifier for determining the degree of amplification of the respective wave, and a corres ionding plurality of wave-control devices connected to the respective amplifiers for modifying each wave in accordance with a differentindication 4. A multiplex transmission system comprising a. source of control oscillations, means for deriving from said control oscillations a plurality of different frequency harmonic oscillations, means for mod ulating each of said harmonic oscillations both in accordance with low frequency variations individual thereto and said control oscillations, and means for simultaneously transmitting. each of said modulated oscillations.

5. The method of transmission which comprises deriving acarrier wave from base frequency oscillations, modulating said carrier waverin accordance with another wave, suppressing all components except oneside band of the modulated carrier wave, deriving other Waves from saidlbase, frequency oscillations and reinforcing said side band with said other Waves.

6. The method of transmission which comprises deriving a carrier wave from base frequency oscillations, modulating said carrier wave in accordance with a signal and said base frequency oscillations, suppressing all components, except one side bandof the modulated carrierwave, utilizing the component of the modulated wave resulting from the modulation of the carrier Wave by the base frequency to reproduce oscillations of the carrier frequency and reenforcing' said side band with said reproduced oscillations.

7. The method of multiplex carrier Wave transmission which comprises producing base frequency oscillations, deriving therefrom harmonic Waves of a plurality of different frequencies, selecting one of said harmonic waves to serve as a carrier wave, modulating said carrier Wave in accordance with another wave, suppressing all components of said modulated wave except one side band, transmitting and receiving said side'band, deriving reinforcing oscillations of said carrier frequency fro-misaid base frequency oscillations, and utilizing said reinforcing oscillations to reinforce said received side band. i

8. The method of multiplex carrier wave transmission which comprises producing base frequency oscillations, deriving therefrom harmonic waves ofa plurality of different frequencies, selecting one of said harmonic Waves to serve as a carrier wave, modulating said carrier Wave in accordance with a signal and with said base frequency oscillations, suppressing all components of the modulated wave except one side band, transmitting and receiving said side band, utilizing the component of the side band produced by the modulation of said carrier wave by the base frequency oscillations to reproduce oscillations of the carrier frequency, and reenforcing said side band by said reproduced oscillations.

9. A transmission system comprising a source of base frequency oscillations, means for deriving a carrier wave therefrom, means for modulating said carrier Wave in accordance with another wave, means for suppressing all components except one side band of the modulated carrier wave, means forderiving reinforcing oscillations from said base frequency oscillations, and means for reinforcing said side band with said reinforcing oscillations.

'lOfA transmission. system comprising a source of base frequency oscillations, means for deriving a carrier wave therefrom, means for mo'dulati-ngsaid carrier wave in accordance with a signal andwith oscillations from 'of said harmonic said source, means for suppressing all components except one side band of the-modulated carrier wave, means for reproducing from a component of said'side band oscillas tions of the carrier frequency, and means for reenforcing said side band with the, reproduced oscillations.

11. A multiplex carrier wave transmission system comprising means for producing base frequency oscillations, means i for deriving therefrom harmonic waves of a plurality of different frequencies, means for selecting one ofsaid harmonic waves to serve as a carrier wave, means for modulating said carrierwave in accordance with another wave, means for supprc ng all components of said modulated wave except one side band, means for transmitting and receiving said side band, means for deriving reinforcing oscillations ofsaid carrier frequency from said base frequency oscillations, and means for causing said reinforcing oscillations to reinforce said, recei ved side band.

12. A multiplex carrier wave transmission system comprising means for producing base frequency oscillations, means for deriving therefron'i harn'ionie waves of a plurality of different frequencies, means for selecting one *aves to serve as a carrier wave, means for modulating said carrier wave in accordance with a signal and with said base frequency oscillations, means for suppressing all components of-said modulated wave except one side band, means fortransmitting and receiving said side band, means for reproducing from a component of the side band reenforcing oscillations of said carrier frequency, and means for, causing. saidreenforcing oscillations to reenforce the received side band.

13. In a multiple signaling system, means to modulate a carrier wave in accor .lance with signals to be transmitted, means for suppressing the unmodulated component of said *arrier wave, means to modulate a carrier wave of a different frequency inv accordance with other signals and'in accordance with a control wave, and a receiving station comprising means operable by. the control Wave modulation for restoring the unmodulated component to the first mentioned modulated wave.

14L. In combination, means for supplying carrier waves, means for modulatingsaid carrier waves by other waves of higher-than the essential speech frequencies, a filter connected to said modulatingmeansto transfer the modulated carrier Wave to a transmitting circuit, said filter h av ing a frequency transmission range lying entirely above the range of essential speech frequencies but including at least a portion of the frequency range occupied by saidmodulated carrier wave, means connect-ed to said lilterfoi: transmitting the modulated energy transferred theretl irough,

and means for receiving and detecting said transmitted Wave. 7

15. The method of signaling comprising deriving carrier oscillations from a base frequency source, utilizing the derived oscillations to produce a signal-modulated wave comprising upper and lower side bands, suppressing all components of the modulated wave except one side band, transmitting and receiving said side hand, producing under control of said base frequency source Waves of the carrier frequency and utilizing the waves so produced to rcenforce the received side band.

16. The method of signaling comprising deriving a carrier Wave from a base frequency source, controlling the carrier Wave in accordance With signals to be transmitted to pro duce a resultant modulated Wave having side bands, suppressing all of the components of the modulated Wave except one side band, transmitting and receiving said side band, utilizing waves from said base frequency source to reproduce at the receiving point a Wave of the carrier frequency and utilizing the wave so reproduced to reenforce the re ceived side band.

17. The method of multiplex signaling, Which consists in transmitting a plurality of currents of different frequencies all harmonic to a certain basic frequency and each modulated according to one or more messages, and receiving these messages by detection with the aid of locally generated currents of the same harmonic frequencies, and synchronizing said locally generated currents by means of one of the said transmitted harmonic frequency currents appropriated to that purpose.

18. In a multiplex carrier current system, a line, means to generate a composite alternating current of a certain fundamental frequency and to separate the component currents of other frequencies to respective channels associated With said line, means at the sending end to modulate one of the higher frequency component currents at the rate of the fundamental, and means at the receiving end governed by the said currents so modulated-to control the generation of a current corresponding to the composite current generated at the other end of the line.

19. In a multiplex carrier current system, a line, means at one station on the line to generate a composite alternating current of a certain fundamental frequency, means to separate its components to various conductors and to modulate them for signaling and specially to modulate one such component regularly at the rate of the fundamental alternating current and to put all the components so modulated on the line, and means at another station controlled by said specially modulated component current to generate an alternating current like that generated at the first mentioned station.

20. In a multiplex carrier current system, a line, means at one station on the line to generate a composite alternating current of a certain fundamental frequency, means to separate its components to various conductors and to modulate them for signaling and specially to modulate one such component regularly at the rate of the fundamental alternating current and to put all the components so modulated on the line, and means at another station controlled by said specially modulated component current to generate an alternating current like that generated at the first-mentioned station, and means to utilize the components of said alternating current generated at said other station for homodyne reception thereat.

In Witness whereof, I hereunto subscribe my name this 15th day of April A. D., 1919.

BURTON W. KENDALL. 

